An inscribed angle in a semicircle is a right angle.
Thales' Theorem
See also
Inscribed Angle, Right Angle, SemicircleExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Thales' Theorem." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ThalesTheorem.html